For world-builders who work upwards, from physical detail to people, there comes a phase in every project when you have to worry about climate: the long-term patterns in weather that will matter to the civilizations of your world. This article is a practical guide to getting yourself a decent, believable climate without enormous amounts of work. You can consider this a first-order approximation of climate; there are many ways to add more detail and realism, and I’ll point some of these out as we go. I often do world-building with the computer, but here I’ll mostly talk about drawing on a paper map, for simplicity.

Climate modeling should come after establishing a rough geography — because it will depend on that — and will allow you to delineate biomes, like tundra or rainforest. By ‘rough geography’ I mean the outlines of continents, and perhaps mountain ranges. Rivers and drainages are nice too, but not critical.

Our Focus: Temperature and Rainfall

Climate is very largely determined by two big factors: temperature and rainfall. At a basic level, we can think about these just in terms of yearly averages. If we wanted to be more sophisticated, we’d start considering seasonality (i.e. variations in both over the course of a year) and variability between years — including from global cycles, like El Nino / La Nina.

So our real goal is modeling temperature and rainfall over the surface of our world. We can handle each somewhat independently, and then look at them together to determine biomes. We can model gradients of continuous change, or delineate boundaries between very different regions, depending on how detailed we’d like to be.
Several aspects of geography affect our variables, which we can work with in turn. We’ll start by looking at two big patterns concerning latitude.

Latitude and Air Flow

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Pattern 1: A Hot Equator

We’ll start with the easy one: the closer you get to the equator, the more sunlight the surface receives, and the warmer it tends to be.

The Earth’s surface also reflects more energy at the equator, but in a less extreme way, so there’s a net gain. The exact pattern is somewhat sinusoidal:

Pattern 2: A Wet Equator

The warming of the equator has a major effect on the movement of air across latitudes, as warm air tends to diffuse outward to the poles. This also influences rainfall, but not in a perfectly obvious manner.

As the sun warms the equator, the air there tends to rise. As it ascends through the atmosphere, it begins to cool down. The water in the air also cools, and condenses out as a liquid — producing heavy rain near the equator. This defines the tropics, with warm and wet conditions, from 0 to about 20 degrees latitude on Earth.

Our moving air is now still warm, but much drier. It will stop rising eventually and begin to spread out, toward the poles. When it gets clear of the rising column of air behind it, it can descend back to the surface, where it will create warm and dry conditions. This happens on Earth between 20 and 30 degrees of latitude — where many deserts lie.

Our air will eventually move back toward the equator, creating a continuous cycle of flow, called the Hadley cells. They’re very efficient at spreading temperature across the planet, which is why deserts remain quite hot, even though they receive less radiation than the equator.

Pattern 3: More Circulation Cells

This pattern of circulation is repeated again, for another band of wet air around 60 degrees, and dry air at the poles. This makes for three regions of air movement in each hemisphere, which define seven major climatic zones.
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These other cells are also caused by the flow of warm air toward the poles, though are more like eddies, than the Hadley cells. They are a good bit weaker as well, which creates less intense rain, and less uniform temperatures in the upper latitudes.

Application: Draw Some Parallels

A good first step then, is to take your map, and draw lines of latitude at and between the 0, 30, 60 and 90 degree marks. (What projection you’re using can matter a lot in precisely how these would be drawn, but let’s not get too technical here.)

This produces the very bare bones of some climactic zones: what could be tropics, and desert, and temperate regions of one kind or another. And all this can change based on later patterns.

Exoplanets and Circulation

Will there always be six circulation cells? Will they always end on the same latitudes? No, it turns out.
The size of the Hadley cells depends on the radius of the planet and its rotation speed. Cells can only get so large, and wind speeds get so high, before they will break up into smaller ones. So a larger planet may demand more cells.

Rotation also breaks up air flow, causing smaller Hadley cells and more secondary cells. An Earth-like planet spinning much slower, say with a day 100 times longer, would have just a single circulation cell in each hemisphere. A day lasting much less, say 15 minutes, would create very small Hadley cells, and presumably many moderate bands of wet and dry climate.

Ocean Currents

Now that we understand something of the movement of air, we can turn to the movement of water. It also redistributes temperature and moisture around the globe.

Pattern 4: Clockwise and Counter-Clockwise Flows

Some of the largest water currents are cyclical ones, which occur near the equator. These are called gyres. They tend to cycle clockwise in the Northern hemisphere, and counter-clockwise in the Southern. They are caused by air currents that cycle in the same fashion.

Why does air move this way? It actually goes back again to the Hadley cells. As the Earth spins, the air above it tends to lag behind. This means that relative to the surface, the air appears to be moving Easterly. Thus do we have jet streams and equatorial counter-currents (shown a few images down) that always flow from West to East.

This deflection of air is strongest near the equator, and weakest at the poles, which means North-South flows are deflected more at lower latitudes. This interacts with the North-South flow of the Hadley cells, deflecting surface winds to arrive partially from the East. The same thing happens with the other circulation cells:

Where these winds fall over water, the ocean currents take on the same directions of flow. So in the Northern hemisphere, water near the equator is pushed Southwest. But it runs up against water coming from the other hemisphere, so begins a cycle, going West and then North — but really Northeast, because of the wind again. This sets up the clockwise and counter-clockwise gyres.

Application: Draw Equatorial Currents

This basic rule can allow us to draw in gyres and other major ocean currents on a map. Let’s make things a little more real now, and introduce an example world to draw on.

Begin by identifying major contiguous regions of ocean between 0 and 60 degrees. Draw a circle in each one. Next, add in some arrows to indicate the direction of flow, such that they correspond with the diagram above: clockwise in the North and counter-clockwise in the South.

These are our gyres. We can now fill in other currents in the rest of the ocean. But these won’t obey the same simple rules. Instead, we want to ensure that where they touch the existing gyres, water’s moving the same way. They do not have to be strictly cyclical, but it’s not a bad place to start:

A few final additions will round out our current map. Right on the equator, and when you get close to the poles, water tends to escape the major gyres (because there is little or no coriolis effect), and simply flow Easterly.

And, of course, these cycles do flow into one another: you’ll need to make smooth connection between them, so water always comes from somewhere, and never goes nowhere.

Pattern 5: Water Brings Temperature and Moisture

The ocean is warmer near the equator. Our currents will move that warm water toward higher latitudes, either Easterly or Westerly. As they brush up against land, the continental air will also warm, changing the local climate. The reverse also holds: where cold water is moved toward the coast from the poles, the land cools. Here’s a close-up from the previous example:

Warm, Southerly water is brought North, and cool Northerly water is brought south. The land on the left will be warmed, while the peninsula on the right will be cooled.

Coasts are generally wetter than the interiors of continents, and this is especially true when warm air flows in from the ocean: it has absorbed oceanic moisture and will tend to rise, creating rainfall. The deep interiors of continents, on the other hand, will not receive as much wet air, making the formation of deserts more likely there, even at higher latitudes — such as the steppe deserts of Eurasia.

Application: Note Warm and Cool Coasts, Dry Interiors

Look at the currents you’ve drawn. Where each one approaches a coastline, consider where it’s coming from, and what kind of air it’s bringing — is it warmer, or colder than the coast would otherwise be? For the major gyres near the equator, this will generally mean warmer East coasts, but not in all cases.
Here’s out example world, roughly colored by relative warmth or cold:

Exoplanet Currents

Much of what we discussed earlier, about circulation cells on exoplanets, will apply here as well. On larger or faster-rotating worlds, the Hadley cells will extend over a shorter range (relatively) and so currents driven by them will also be smaller. This leaves more room for smaller circular eddies, and for long-distance currents.

Topography

Without getting into too much detail, I think we do have some time for mountains and elevation.

Pattern 6: Elevation Brings Cold

A higher elevation means thinner air, and that means colder air. A kilometer of extra height typically means a 6.4 degree drop in temperature (Celsius). And this will mean that a temperate climate can be found in, say, the Southeast United States — just go to the Blue Ridge mountains, and you can pretend you’re in New England (sort of).

If you have a digital elevation map of your world, it’s easy to model this change everywhere. If you have a hand-drawn map, or don’t have all the topography yet, you can make do with the major effects on mountain ranges.

Pattern 7: Mountains Deflect Air

This happens in two ways. First, air movement can simply be shifted a little bit, as it glances off a very large land mass — before proceeding to deliver its temperatures and humidity somewhere else.

Second, air can be forced up over a mountain range. As this happens, it will expand and cool, dumping precipitation on the windward side of the mountain range. On the other, leeward side, the air will descend and be less prone to produce rain — it’s also lost a lot of its moisture already. This “rain shadow” effect can be quite pronounced.

Next Steps: Identify Climate Zones

One example is the Köppen classification, which has several levels of detail. The first is very simple, with five zones based mostly on temperature. Here’s the Earth:

Here is our example world, with a very crude overlay of these zones (with no mountains). The key latitudes to keep in mind are those we drew earlier, at 15, 45 and 75 degrees.

Starting to Get Ecological

Most world-builders are probably also familiar with Whittaker diagrams. Created by the ecologist Robert Whittaker, these show the biome that’s created with a given combination of temperature and rainfall. They’re certainly over-simplifications, but extremely helpful for actually drawing biological regions on your map.

These are based primarily on vegetation, but that’s a good starting point, and vegetation does define a lot about ecological communities. The upper right half is mostly empty, since these climate combinations do not exist on Earth — remember that cold air cannot hold moisture.

(You will sometimes find cruder, squared-off versions of Whittaker diagrams, which are a bit easier to program into a simulation, but may produce artificial-looking results, as you might expect.)

Of course, this mapping depends on plants that are somewhat similar to those on Earth; not in detail, necessarily, but which “make their living” in similar ways, and form similar communities. If you want to get more exotic, you can easily break that assumption, but then you’ll need a custom Whittaker diagram of your own.